1. Field of the Invention
The present invention relates to a fixing apparatus for fusing a developer to a transfer material, which is provided in an image forming apparatus using an electrophotographic process to form an image on a transfer material, such as a copier and printer.
2. Description of the Related Art
An image forming apparatus such as an electric copier has a fixing apparatus for fusing a heated and fused developer image to a paper sheet by applying pressure.
As a method of heating a heating member of a fixing apparatus, induction heating is available. Induction heating is a method of generating a magnetic field by applying a predetermined electric power to a coil to generate a magnetic field and generating a predetermined amount of heat in a heating member by Joule heat generated by an eddy current generated by the magnetic field.
For example, Jpn. Pat. Appln. KOKAI Publication No. 2001-235962 discloses a fixing apparatus using an induction heating method, in which a coil having an area where a heating member contacts a paper sheet is arranged opposite to a divided heating area, is divided into a predetermined number of coils according to the size of paper, and placed outside the heating member.
Further, Jpn. Pat. Appln. KOKAI Publication No. 2000-206813 discloses a fixing apparatus using an induction heating method, which has a plurality of exciting coils, and controls the amount of current supplied to the exciting coils except a first exciting coil according to the amount of current supplied to the first exciting coil.
Further, Jpn. Pat. Appln. KOKAI Publication No. 7-295414 discloses a fixing apparatus using an induction heating method, which has a plurality of coils placed outside of a heating member, having an area where a heating member contacts a paper sheet, is arranged opposite to a divided heating area, according to the size of paper heated by the heating member, and a current is supplied independently to the plurality of coils.
In the fixing apparatuses using the induction heating method as disclosed by the above three patent publications, a heating member having a very high heating efficiency is heated very quickly, and if it is heated in the state not rotated, the area near the part opposite to the exciting coil is locally heated.
Further, Jpn. Pat. Appln. KOKAI Publication No. 2002-40839 discloses a fixing apparatus which has a fusing belt heated by a heating roller heated by an induction heating method, and a detection means which detects movement of the fusing belt in the rotating direction.
Further, Jpn. Pat. Appln. KOKAI Publication No. 2002-82549 discloses a fixing apparatus, in which a part of a belt contacting a passing paper sheet heated by a heating member is separated from a part cooperating with a pressing member to supply a predetermined pressure to a paper sheet, and the heating member starts induction heating after the belt is rotated.
Among the fixing apparatuses using an induction heating method, the fixing apparatus which uses a plurality of coils for induction heating may have a weak magnetic field strength supplied to the area adjacent to the coil, compared with the magnetic field supplied close to the center of the coil. In this case, the magnetic field intensity varies in the length direction of the heating member, and the heat amount changes depending on the positions of the heating member.
Therefore, the distribution of temperatures in the length direction of the heating member becomes nonuniform, and the heat value supplied to the developer on a paper sheet becomes unstable.
In a heating roller with a thin metallic layer noticed in recent years, the temperature difference that occurs particularly among a plurality of coils becomes a problem.
As for a detection means for detecting an abnormal temperature, it is demanded to detect a temperature at a predetermined position heated locally by an exciting coil.
However, in a fixing apparatus which does not contain an exciting coil and a means for detecting an abnormal temperature, because the heating member is filled inside, it is physically difficult to place a temperature detection means at a predetermined position that is locally heated, for example, between a coil and a heating member.
Besides, there is a problem in the fixing apparatus which uses a heating member with a filled inside. As an abnormal temperature detection means for detecting an abnormal temperature is arranged close to an exciting coil, a magnetic field is not evenly supplied from the exciting coil to the heating roller, and the temperature is not held uniform in the rotating direction of the heating member.